The invention is concerned with an improvement of an electrographic printer means for printing web-shaped recording media of different web width according to WO 94/27193. The printer means disclosed therein has an electrographically working intermediate carrier, for example, a photoconductor drum, having a usable width corresponding to twice the width format of a standard form according to DIN A4 or twice the width of standard letter size format. The further units, such as the fixing station, the developer station, the cleaning station, etc., are likewise designed for this usable width.
Various operating modes are possible with this known printer means. In what is referred to as the simplex mode, thus, a recording medium having up to twice the width of a DIN A4 sheet or of a sheet in letter size format can be printed in a traditional form. In a parallel simplex mode, two narrow recording media arranged side-by-side, for example having a width according to DIN A4, can be conducted through the printer means and be printed in juxtaposed position.
In a further operating mode, the single-color duplex mode, the web of the recording medium is turned over during transport through the printer means, so that two web sections derive in a first web section, the front side of the web resides opposite the transfer printing location of a printing unit, whereas the back side of the web in a second web section is simultaneously printed at the same transfer printing location. A two-color duplex mode is also possible by employing differently colored ink particles in different developer units of the printing unit.
In another operating mode, the two-color simplex mode, the web is offset parallel by at least one web width during transport in the printer means, and the offset web sections are conducted past the transfer printing station in common in juxtaposed position. In the first pass of the web past the transfer printing location, image and text elements are printed with a first color; in the second pass of the web with offset, image and text elements are printed with the second color.
The printer means disclosed by WO 94/27193 works according to the principle of electrophotography, whereby a photoconductor on which a latent charge image corresponding to the print format to be printed is applied with the assistance of a light source, for example a laser or an LED line, is employed as intermediate carrier. Ink particles of a desired color are transferred onto the photoconductor within a developer station arranged close to the photoconductor and the charge image is inked with toner particles. The developer station is followed by the printing unit in which a corona device, arranged at a distance from the photoconductor, transfers the toner image onto the endless carrier material passing through between photoconductor and corona device with an electrical field. To this end, the air between the corona device and the endless carrier material is ionized by high field strengths, as a result whereof charge carriers are generated on that side of the endless carrier material facing away from the photoconductor. An electrostatic force field thereby arises between endless carrier material and photoconductor, as a result whereof the toner image situated on the photoconductor is transferred onto the endless carrier material.
Since the printer of the species initially cited can print web sections of different width from endless carrier material, only a section of the photoconductor corresponding to the widths of the web section is covered by the endless carrier material when printing a web section of less width, whereas the section that is not covered is exposed to the electrical field of the corona device unprotected. Due to the strong electrical field, the section of the photoconductor that is not covered is highly charged compared to the section covered by the endless carrier material, so that an extremely great difference in potential arises between the photoconductor sections that are covered by the endless carrier material and the photoconductor section that is not covered. These great fluctuations in potential deteriorate the quality of the print image, which appears, for example, given gray rasters or large-area print images, which make a non-uniform impression. Further, a uniform charging of the photoconductor over its entire width is not longer possible due to the great differences in potential, and the photoconductor is made unusable for a longer time due to the uncontrolled charging.
Width-adjustable transfer coronas are notoriously known. Thus, JP-A-4-174480 provides a length-adjustable corona wire that is stretched via an electrode roller and an insulator roller and is moved together with a non-conductive wire. JP-A-58/172667 shows two predetermined sections wherein the field width can be registered via an opposing voltage applied to shielding lattices. U.S. Pat. No. 3,578,970 shows corona wires that are stretched between a stationary block and a displaceable block. The field width set corresponds to the spacing between the two plates secured to the blocks.